Originally Posted By: "tayedrumma"
would it be possible to have two? one in the standard location and another facing out the back of the screen

On a Powerbook G4 15 inch Aluminium screen module, I don't think there is even space for one in the standard location, due to the location of the latch. You would probably have to mount one off to the left or right of the latch, and mount the other one on the opposite side.

Photobooth can handle two iSights connected though (well, I've only tested with one firewire + one USB), it lets you switch between them with a control that is only visible if two are present.